Donald Trump withdrew Tuesday from the final Republican presidential debate before the Iowa caucuses after the sponsor, Fox News, mocked him for challenging the credibility of its anchor Megyn Kelly.

It was an extraordinary breach between the front-runner for the GOP nomination and a conservative network that exerts enormous influence over the Republican Party. It was also emblematic of the high-risk and unpredictable approach that the New York billionaire has taken in his run for president.

In a combative news conference in Marshalltown, Iowa, Trump said his decision was “pretty close to irrevocable,” but released a statement later saying it was definite. He took on Fox News Chairman and Chief Executive Roger Ailes by name, a step that few if any other Republicans would dare to take.

"I don't know what games Roger Ailes is playing, but what's wrong over there?" Trump said.

The former reality television star now plans to stage an event to raise money for veterans at the same time as the Fox News debate Thursday evening in Des Moines.

"They can't toy with me like they toy with everybody else," Trump said. "So let them have their debate, and let's see how they do with the ratings."

Trump's blowup came a few hours after Fox News reacted sarcastically to his threat to boycott the debate because of Kelly's role as a moderator.

“We learned from a secret back channel,” the Fox statement said, “that the Ayatollah and [Russian President Vladimir] Putin both intend to treat Donald Trump unfairly when they meet with him if he becomes president — a nefarious source tells us that Trump has his own secret plan to replace the Cabinet with his Twitter followers to see if he should even go to those meetings.”

Trump's feud with Kelly started at the first GOP debate last August, when she asked him about his history of calling women "fat pigs, dogs, slobs and disgusting animals." She questioned whether that sounded like the temperament of a president and asked how he would respond to charges he was part of "the war on women."

On Tuesday, Trump called Kelly a "lightweight" and a "third-rate reporter" who was biased.

Kelly told viewers on her prime-time show, "The Kelly File," that she was simply doing her job.

"Mr. Trump has repeatedly brought up that exchange as evidence of alleged bias on my part," she said. "I maintain it was a tough but fair question, and we agreed to disagree."

On Tuesday night, Fox News put out another statement saying Trump had been "viciously" attacking Kelly since August.

"Capitulating to politicians' ultimatums about a debate moderator violates all journalistic standards, as do threats, including the one leveled by Trump's campaign manager Corey Lewandowski toward Megyn Kelly," the statement said. "In a call on Saturday with a Fox News executive, Lewandowski stated that Megyn had a 'rough couple of days after that last debate' and he 'would hate to have her go through that again.'

"Lewandowski was warned not to level any more threats, but he continued to do so. We can't give in to terrorizations toward any of our employees."

Trump spokeswoman Hope Hicks did not respond to a request for comment.

Trump's exit from the debate upended the Republican contest just six days before Iowans vote in the year's first presidential nominating contest. His absence on the stage with seven opponents Thursday is the latest of many unorthodox maneuvers that have helped Trump dominate the campaign.

It also heightened the volatility of the race. Trump holds a wide lead in national polls of Republicans, but in Iowa, he is locked in a tight and hard-fought battle against Ted Cruz for first place. The Texas senator’s strength among evangelicals and tea party followers poses a threat to Trump, whose momentum may break if he loses Iowa.

Trump rivals criticized his decision. Cruz offered to debate him one-on-one, saying a candidate afraid of tough questions from Kelly could never take on Putin or Hillary Clinton, the Democratic front-runner.

Kelly's question about women led Trump to call her a bimbo. On CNN, he described her as "having blood coming out of her eyes, blood coming out of her wherever." Many thought the statement referred to menstruation, which Trump denied.

On Tuesday, Trump posted a video on Instagram, asking followers if he should participate in the debate. "Megyn Kelly is really biased against me," he said. "She knows that. I know that. Everybody knows that. Do you really think she can be fair at a debate?"

Trump had threatened to boycott previous debates. He requested a $5-million charitable contribution to veterans' groups in return for his appearance on a CNN debate. The network never considered it, and Trump still showed up.

"Megyn Kelly is an excellent journalist and the entire network stands behind her — she will absolutely be on the debate stage on Thursday night," Ailes said in a statement to The Times.

Fox News averaged 24 million viewers for the August debate, a cable-ratings record outside of sports programming.

"Let's see how much money Fox is going to make on the debate without me, OK?" Trump said Tuesday.

Kelly, one of Fox News' biggest stars, has not responded to Trump's comments. In an interview with The Times before the events of the last two days, she said she did not think of the upcoming debate as a rematch with Trump, and maintained that viewers wouldn't see any change in her approach.

The ratings of Kelly's show have risen since August. She has also done a magazine cover shoot with Vanity Fair and signed with talent agency CAA.